Shifting assemblies in armature and clutch mechanisms generally require a motion-transmitting device which steps down the rotation rate of the primary power source and which is insertable into an environment having a fluid pressure different from the internal pressure of the motion-transmitting device, e.g. into a pipe of a clutch housing. Motion-transmitting devices of the harmonic-drive type for use in these applications are described in German patent document Ser. No. 1,135,259 and also in the text "Gear Handbook," edited by Darle W. Dudley and published by McGraw-Hill (1962).
It is mentioned in the German patent document No. 1,135,259 that the use of ball bearings in a harmonic-drive device is essential for lowering the friction coefficients and for increasing efficiency. The balls are seated with a narrow tolerance between the inner and outer races, the spreading of the inner race along one axis causing the outer race to assume a substantially elliptical form. Upon a rotation of the inner race, two waves or moving lobes are produced in the outer race. In order to ensure the driving of a secondary or motion-output component disposed on the other side of the bearing assembly from the driving or input component, more specifically on the other side of a flexible wall member from the bearing, it is suggested to use bearings with at least 36 balls.
The outer race of the wave-generating bearing must be sufficiently thin to prevent loading beyond its elastic limit. The spreading of the bearing along one axis and the compression along an orthogonal axis results in increased play of balls at the troughs of the generated waves. The inner race must be supported on each side of the wave crests in the event of a load arising on the bearing there.
The disposition of the balls and the races for different loading conditions and different diameters has been problematic. In spite of the general durability of strength or harmonic-drive assemblies, it has proven necessary to limit the application thereof in situations where additional stresses might arise in the flexible wall member. Inwardly projecting teeth of a ring gear meshing at points spaced from the wave crests with teeth on the outer surface of the wall member are frequently apt to distort or destroy the wave shape under increased loading conditions, this disadvantageous result being more likely at higher operating temperatures.
The possible distortion or destruction of the wave shape in the flexible wall member has led to the proposal that this member be supported or braced on the side of the wave generator. Ball bearings provided for this purpose are ineffectual in equalizing support in different regions of the wall member. Because of excessive strains and raised external pressures, the need for sufficient lubrication is readily apparent.
The utilization of conventional harmonic-drive assemblies is, therefore, very limited in applications involving elevated external temperatures and pressures, extended operation times and sudden mechanical loadings or shocks. The use of a conventional reinforced harmonic-drive assembly is further reduced by the costs of building in the bracing elements. Because these bracing bearings are subjected to unusually high loads, their expected life span is short, which contributes even further to increased costs and reduced efficiency.
Conventional harmonic-drive devices are generally limited to situations involving low external pressures, because higher pressures distort or bend the flexible wall members and thus quickly destroy the devices.